The invention relates to a method for automatically measuring the atomic absorption of gaseous measuring samples generated successively from a series of liquid samples, in which method within one measuring cycle a proportionate amount of liquid sample is supplied to a reactor through a sample input for reaction therein with proportionate amounts of at least two liquid reagents to form said gasous measuring sample which is conveyed to the measuring cell of an atomic absorption spectrometer by means of a carrier gas stream passing through said reactor.
The invention also relates to apparatus for carrying out the aforementioned method and comprising a reactor with carrier gas input and outlet lines, with a sample input and with at least two reagent inputs, sample taking means and supply means adapted to supply liquid sample and at least two liquid reagents to said reactor, and a heatable measuring cell placed in the optical path of rays of an atomic absorption spectrometer and connected to the reactor through the inert gas outlet line.
Methods and apparatus of such kind for the automatic determination of arsenic, antimony, or selenium in liquid samples known (P. D. Goulden, P. Brooksbank, Analytical Chemistry, Vol. 46, No. 11, Sept. 1974, pages 1431 to 1436; F. D. Pierce, T. C. Lamoreaux, H. R. Brown, R. S. Fraser, Applied Spectroscopy, Vol. 30, No. 1, 1976; pages 38 to 42). According to said known methods liquid sample and liquid reagents consisting of an aqueous acid on the one hand and either of an aqueous solution of sodium borohydride or an aluminum slurry on the other hand are fed by means of a multiple proportionating pump to a reactor in which the liquids are mixed and reacted chemically. The reactor is provided with input and output lines for inert gas which passes through the reaction mixture and serves as a carrier gas for the volatile hydrides formed in the reactor. The reactor is connected to a heatable stripping device in which the reaction mixture and the hydride containing inert gas are separated. Subsequently, the inert gas stream containing the hydride passes through a wash column and then enters a heatable tube placed in the optical path of rays of an atomic absorption spectrometer. The tube is heated to such a temperature that the gaseous hydride contained in the inert gas stream is decomposed to form atoms of the respective element to be determined by measuring the atomic absorption thereof.
In the known apparatus a separate device for taking samples of the liquid to be investigated ensures that no carry-over of material occurs between different samples. The whole apparatus and the method, however, are designed in such a way that mutual contamination of the liquid samples and of the gasous measuring samples cannot be exluded to the desired degree of perfection. The flow conduits and the apparatus components are of such size that only relatively large amounts of liquid samples will ensure that perfection which perfection is not obtained if smaller amounts of liquid sample are used.
One object to be achieved by the invention, therefore, is to provide a method and apparatus of the initially mentioned kind which are also suited for processing small amounts of liquid sample.
A further object to be achieved by the invention is to provide an apparatus of the initially mentioned kind having small-sized receptacles and flow conduits to enable the processing of small amounts of liquid sample, for example similar to those provided in clinical investigations.
A still further object to be achieved by the invention is to design the aforementioned method and apparatus in such a way that blank and reference measurements can also be taken without any further modifications.
According to the method of the invention the foregoing and other objects are achieved by feeding uniformly and continuously liquid reagents to the reactor during a first and a second time period of the measuring cycle and liquid sample during the second time period, both in countercurrent to a carrier gas stream, and by emptying and washing the reactor during a third through a sixth time period of the measuring cycle. During the first time period a blank sample may be supplied uniformly and continuously to the reactor at the same flow rate as the liquid sample is supplied during the second time period to conduct the blank measurement.
To perform the washing, wash liquid may be taken from a wash liquid receptacle during the third time period of the measuring cycle and supplied to the reactor through the sample input during a fourth time period.
The time periods of the measuring cycle may be of the same duration.
Since the different liquids are supplied in appropriate but over different time intervals small amounts of liquid sample can also be processed with sufficient safety and assurance against carry-over. Further, by washing the reactor the occurrence of mutual contamination of the gasous measuring samples originating from different liquid samples is prevented. Countercurrent flow of the inert gas with respect to the reaction mixture in the reactor will effect particularly efficient and complete removal of the gasous measuring sample from the reaction mixture immediately after formation without requiring a separate stripping device. Separate supply of the liquid reagents to the reactor also enables a blank measurement to be regularly taken before sample each measurement. This is of particular importance in the field of trace analysis.
In the method according to the invention, an addition liquid is fed uniformly and continuously to the reactor during further discrete time periods of the measuring cycle extended by said further time periods intermediate the second and the third time period thereof, while liquid reagents are being supplied to the reactor during said first and said second time period and during said further time periods. Said further time periods maybe of equal duration between themselves and of the same duration as the time periods of the measuring cycle.
The method according to the invention will thus enable blank and reference values to be taken for each respective liquid sample. The type of method used, again, will ensure that no carry-over occurs and that satisfying blank and reference values are obtained without large volumes of blank and reference samples being required.
In the case of gaseous measuring samples like mercury vapour the atomic absorption of which can be measured without applying thermal decomposition, the method in accordance with the invention is conducted in such a way that the flow of inert gas is interrupted before the second time period begins, the gas mixture enclosed in the reactor and the measuring cell being circulated in a closed loop during the second and third time period.
The aforementioned objects are achieved by an apparatus according to the invention comprising a reactor in the form of a reaction vessel with a packing, the reaction vessel having an inert gas input line connected to the lower end, an inert gas outlet line connected to the upper end thereof and a sample input and reagent inputs located just above the packing, comprising further sample taking means including a sample carrier adapted to be advanced in steps and to receive a series of sample receptacles and a wash liquid receptacle, said sample carrier being movable between a first position, in which one of the sample receptacles is placed in a withdrawal position during the first time period of the measuring cycle, and a second position, in which the wash liquid receptacle is placed in the withdrawal position during the third time period of the measuring cycle, and also including a withdrawal tube adapted to be introduced temporarily into the respective receptacle placed in the withdrawal position, said apparatus also comprising a withdrawal tube connected to the sample input, conduits to connect each of the reagent inputs to a respective reservoir and controlled supply means each governing the sample input and the conduits, respectively, and finally comprising control means designed to control the sample taking means and said supply means in accordance with the method steps during the time periods of each measuring cycle.
Thus, the liquid flow and the inert gas flow through the packed reaction vessel in countercurrent fashion and further 10 mixture of sample and reagent liquids and complete removal of the gaseous measuring sample from the reaction mixture in a particularly efficient manner immediately after formation. No separate stripping and washing devices are required. Connecting the sample input to the withdrawal tube and the reagent inputs to conduits leading from respective reservoirs together with providing controlled supply means governing each enables the supply of even small amounts of liquids and the reactions to be conducted in such a way that mutual contamination of liquid samples or of gaseous measuring samples cannot occur. This advantage is assisted by providing a wash liquid receptacle on the sample carrier in the withdrawal positions of which washing liquid is taken up by the withdrawal tube and delivered to the reactor.
In accordance with the invention the reaction vessel comprises a funnel-shaped discharge section and a packing formed by an insert body having a helical face directed towards the discharge section and having a baffle plate extending upwardly from said helical face to the level of the sample and reagent inputs. In a more simple design the packing is formed by Raschig rings filling the reaction vessel to the level of the sample and reagent inputs. By means of the packing, the liquids supplied to the reaction vessel will become distributed over a large surface area which particularly intensifies the displacement of gases from the liquid reaction mixture by the countercurrent flow of inert gas.
In the apparatus according to the invention the sample carrier is a turntable stepwisely rotatable about its axis on a base plate movable between stops defining the first and the second position of the sample carrier, the wash liquid receptacle being placed on the base plate. The turntable may have a serrated rim and the base plate may be eccentrically pivotably supported on a fixed instrument plate, the base plate having a first ratchet and instrument plate having a second ratchet, both said ratchets being arranged for alternating engagement with said serrated rim so that said turntable will be automatically advanced one step on conclusion of the first time period of the measuring cycle. By combining such a turntable, which is suitable for receiving a great number of sample receptacles, with a pivotable base plate rotating and pivoting movements are favourably combined to bring either the sample receptacle or the wash liquid receptacle into a position for withdrawal of the respective liquid. By means of the interaction of an external serration with appropriately positioned ratchets the pivoting movement of the base plate is combined with the turntable advancement.
According to the invention the base plate may also be arranged for displacement between two stops on a bearing, a return spring acting upon the base plate being secured to one of the stops. Such an arrangement is more favourable with respect to the forces to be provided as compared to pivoting the base plate about an eccentric fulcrum.
In the apparatus according to the invention, the controlled supply means are piston pumps each of which is connected to the sample input or a reagent supply conduit, respectively, intermediate two controlled valves. By combining the piston pumps and controlled valves the piston pumps, which may be connected to the control means in a simple manner, are actuated either to take up the respective liquid or to supply the same to the reaction vessel. Piston pumps have been proven to be particularly useful for conveying small proportionate amounts of liquids. Alternatively, the inert gas source may be connected to a closed reservoir via a service line provided with a first pressure regulator and to the inert gas input line via a second pressure regulator, the input line having two branch conduits including in each an adjustable flow restrictor and being connected to the reaction vessel, the service line and the reagent supply conduit passing through the reservoir closure with the reagent supply conduit extending to the bottom of said reservoir. The supply means may comprise two controlled valves adapted to be actuated in anti-phase relationship one of which is positioned in the service line and the other one in one of the branch conduits. Thus the piston pumps and controlled valves are replaced by pneumatic supply means operated by the inert gas passing through the apparatus and serving as a carrier gas for the gaseous measuring sample. A conduit including an adjustable flow restrictor and a flow meter and interconnecting the branch conduits may be provided to monitor the flow of inert gas during operation of the apparatus.
In and apparatus according to the invention, the wash liquid receptacle is designed as an overflow vessel connected to a wash liquid reservoir by a conduit to which a controlled piston pump is connected intermediate two controlled valves, the piston pump being adapted to be controlled by the control means in accordance with the method steps during the time periods of the measuring cycle. The overflow design of the wash liquid receptacle enables interior and exterior washing of the withdrawal tube before the wash liquid is taken up and delivered to the reaction vessel.
The discharge section of an reaction vessel in the apparatus in accordance with the invention comprises a drain line terminating above a drain liquid container and governed by drain means adapted to be controlled by the control means in accordance with the method steps during the time periods of the measuring cycle. Thus it will be achieved that the reaction vessel will only be emptied after complete reaction of the reaction mixture and after complete removal of the gaseous measuring sample by the inert gas. The drain means comprises a controlled piston pump connected to the drain line intermediate two controlled valves. Thus the drain means may be controlled by the same means controlling the supply means for the different liquids. Alternatively, the drain means may comprise a controlled valve which is possible in cases in which the reaction mixture forms a freely flowing liquid.
In a modification of the apparatus according to the invention a reservoir of a blank liquid free of the element to be determined and similar to the liquid sample is provided and connected to the reaction vessel via a blank input line governed by supply means which are adapted to be controlled by the control means in accordance with the method steps during the time periods of the measuring cycle. Thereby, blank liquid will be supplied to the reaction vessel together with the reagent liquids during the first time period of the measuring cycle. In such a way errors in determining the blank value are avoided which otherwise would occur because of the different liquid volumina introduced into the reaction vessel during the first and during the second time period of the measuring cycle.
To perform reference measurements there is provided in an apparatus according to the invention at least one further reservoir for an addition liquid and one further supply line connected to one further input at the reaction vessel and governed by further supply means adapted to be controlled by the control means in accordance with the method step during the further time periods of the extended measuring cycle. The further reservoir may be placed on the base plate of the sample carrier, the base plate being movable at least into a third position, and a further withdrawal tube connected to the further supply line may be arranged for temporary introduction into the further reservoir in the third base plate position.
To conduct the measurement in a circulation mode the reaction vessel, the inert gas outlet line, the measuring cell and the inert gas input line of the apparatus according to the invention form a closed loop comprising a pressure relief valve downstream from said measuring cell and a circulation pump, a junction with a conduit leading from the inert gas source and governed by a controlled valve formed downstream from the circulation pump, said circulation pump and the valve being adapted to be controlled by the control means in accordance with the method steps during the second and during the further time period of the measuring cycle.
According to the invention, the control means comprises a synchronous motor and a cam shaft driven thereby, the cam shaft including control cams drivingly connected to the supply means and also including additional control cams drivingly connected to the sample taking means, the time of revolution of the cam shaft being equal to the duration of one measuring cycle. Thus, a simple and effective central control of the entire apparatus is achieved. One first additional control cam may be drivingly connected to actuating means acting on the withdrawal tube and a second additional control cam may be drivingly connected to a control lever acting on the base plate against the return spring. In such a way, actuation of the withdrawal tube and movements of the base plate carrying the turntable are most simply effected by the control means. The actuating means for the withdrawal tube comprises a lever system in driving engangement with the first additional control cam movable normally with respect to the plane of the sample carrier. A simple and reliable driving connection is thus established between the control means and the withdrawal tube. Alternatively, the actuating means comprises a pneumatic cylinder including a piston which acts on the withdrawal tube, the pneumatic cylinder being connected to the inert gas input line via a valve controlled by the first additional control cam. In such a way the already present inert gas serving as carrier gas may be utilized for pneumatically actuating the withdrawal tube by the control means.
In an apparatus according to the invention, the cam shaft may be provided with a first further control cam drivingly connected to the piston pump for wash liquid. Thus washing of the apparatus is being controlled centrally, also. To realize pneumatic actuation of the piston pump for wash liquid, the first further control cam may alternatively be in driving connecting to a valve controlling a pneumatic cylinder the piston of which acts on the piston pump and which is connected to the inert gas input line.
A second further control cam may be provided in the apparatus in accordance with the invention, the second further control cam being drivingly connected to the drain means. Thus, draining the reaction vessel will be centrally controlled, too. The second further control cam may form a control plate acting on the piston pump of the drain means and providing a mechanical driving connection of the cam shaft and the drain means. Alternatively, the second further control cam may act upon a control valve governing the control line of a pneumatically controlled drain valve which control line is connected to the inert gas input line. In the case of reaction liquids which will flow freely from the reaction vessel there is thus provided a particularly simple central control of the drain means by employing the available inert gas.
Specific embodiments of the invention are illustrated in the drawings and will be described with reference thereto in detail hereinbelow.